An apparatus commonly used in biomedical and analytical applications is a probe for aspirating a liquid from a container. The container is often a cuvette which is transported to an aspirating station where the probe is electromechanically lowered into the liquid and a syringe pump turned on to take up a measured amount of the liquid. In such applications, it is important that the control system know when the probe touches the surface of the liquid, so the downward motion can be automatically stopped and the syringe pump turned on. It is desirable that such determination be made electronically. Since, most liquids of interest (e.g., electrolytes, blood, plasma, water and the like) are electrically conductive, they can function as one plate of a capacitor, with the cuvette, test tube or other liquid receptical (hereinafter "tube") as a dialectric and system ground immediately outside the tube. Thus, measuring the capacitance at the tip of the probe is an excellent technique for determining when the probe is above versus when it has made contact with the surface of the liquid sample.
The aspirating tube is usually made of flexible plastic material with a small bore hole running the length thereof. The aspirating tube is placed in a hollow metal tube that protects the plastic tube and gives it rigidity. This concentric arrangement, the non conducting plastic aspirating tube within a metal tube, extends within a cylindrical metal housing which is coupled to a gear drive or other precision motion drive that mechanically drives the probe into and out of the liquid sample. Both the housing that holds the aspirating tube rigid and the driven outer housing are typically made of stainless steel.
In the above arrangement the tip of the metal rigidizing tube is used as the capacitance measuring point. The tip is typically connected to an oscillator the frequency of which changes with changes in capacitance at the tip. When the probe is not in the liquid, the oscillator frequency is due almost entirely to the "parasitic" or "stray" capacitance at the tip. When the probe is in the liquid, it senses the capacitance of the liquid to ground through the tube which changes the frequency of oscillation.
The capacitance of the liquid to ground depends upon the contact area between the liquid and the tube, which will be small for small volumes of liquid. Accordingly, the capacitance for small volumes of liquid may be small compared to the "stray" capacitance. For at least one prior art probe, the stray capacitance is as large or larger than the capacitance of the liquid to ground. Furthermore, because the motion drive causes slight shaking and vibration of the probe as it is driven down toward the liquid, the value of the stray capacitance in that prior art probe is unstable and subject to frequent changes, which changes can be relatively large.
Another potential source of stray capacitance involves the asperated ionic fluid functioning as one plate. Since this capacitance can vary with the type of fluid in the probe (not being present, for example, if the fluid in the probe is non-ionic or if there are air bubbles in the probe separating fluid layers), with vibration or other movement of the fluid and with other factors, this stray capacitance is both not well-defined and unstable. Under worst case conditions, it may also be relatively large.
These stray capacitances make detection difficult, since, to reliably measure changes in capacitance due to the probe entering the liquid, all other capacitors in the circuit must either be small compared to it, or invariant so they can be differentially eliminated. Ideally, such stray capacitances are both small and invariant. The prior art transducer meets neither of these criteria.